The present invention relates to Stirling engines and more particularly to heat exchanging elements thereof which are formed of platelets.
The basic concept of the Stirling engine dates back to the developments of Robert Stirling in 1817. Over the years, numerous applications for the Stirling engine have been investigated and evaluated. For example, one potential use of the Stirling engine is in automobiles and the like as a prime mover. In addition, the Stirling engine may be used as an engine power unit for hybrid electric applications. Other potential applications are the use of the Stirling engine as an auxiliary power unit and the use of the Stirling engine in marine applications and solar energy conservation applications.
Stirling engines have a reversible thermodynamic cycle and therefore can be used as a means of delivering mechanical output energy from a source of heat, or acting as a heat pump through the application of mechanical input energy. Using various heat sources, mechanical energy can be delivered by the engine. This energy can be used to generate electricity or be directly mechanically coupled to a load.
One of the disadvantages of current Stirling engines is their inefficiency due to the presence of dead volume of a working gas and the overall volumetric size of a burner device of the heat exchanging assembly. A heat transfer system utilizes heat transfer from the burner device to the working gas to cause a piston to be displaced as the working gas expands under heat and then compresses (contracts) upon cooling of the working gas. One conventional burner device is an apparatus in which air and fuel are injected into the burner device and then ignited to cause heat to be generated. The working gas is carried within a plurality of heater tubes, which are positioned proximate to the burner device so that heat is transferred from the burner device to the working gas flowing within the heater tubes.
One end of each heater tube is in communication with a piston chamber which houses one or more pistons and the heated, expanded working gas causes displacement of the one or more pistons within the piston cylinder. The one or more pistons are operatively connected to other working mechanical components for moving a drive member, such as a crankshaft, to cause mechanical energy to be delivered by the engine.
Because a single burner device is used to generate and effectuate heat transfer to the working gas flowing within a number of heater tubes, heat is often not evenly distributed to the working gas within the heater tubes. The burner device in conventional devices often has a length of 14 inches or greater for a 3-kilowatt Stirling engine and the length of each heater tube from the piston cylinder to a point proximate to the burner device is about 6 inches or more. The gas therefore must travel 6 inches up the heater tube and then 6 inches back down the heater tube to the piston cylinder after it has been heated. The associated disadvantage of such a system is that conventional heater tubes usually contain a dead volume of working gas. This refers to the volume of working gas that has not moved out of the heater tube during the expansion/compression combustion process. In other words, this constitutes a volume of stagnant working gas. This results in inefficient heat transfer from the burner device to the working gas and in turn leads to inefficient operation of the Stirling engine itself.
In addition, due to the typical size of the burner device, the burner device first heats a significant volume of air before heat transfer occurs to the working gas. This results in a considerable amount of energy being consumed before the working gas is heated and as a result, the working gas is exposed to less heat due to the inefficiencies of the burner device. In other words, a lot of the heat produced by the burner device does not get transferred to the working gas.
Accordingly, there is a continuing need to design a more efficient heat transfer manifold for use in a Stirling engine.
The present invention is directed to a heat exchange manifold for use in a Stirling engine. According to the present invention, the heat exchange manifold is provided using a platelet construction. More specifically, the heat exchange manifold is formed of multiple platelets that are stacked and joined together. A platelet device is a device, which is designed to control and manage fluid flow and is constructed of individual layers (called platelets). The platelet construction of the heat exchange manifold provides integrated fluid management (IFM), which advantageously permits the Stirling engine to run more efficient because the heat exchange and combustion process are improved.
The platelets have openings and conduits formed therein which are orientated relative to one another to form the elements of the heat exchange manifold. For example, the manifold includes a combustion chamber having fuel and air intake conduits for delivering fuel and air to the combustion chamber and an exhaust conduit for venting exhaust gases and the like from the combustion chamber. The manifold also includes a working gas circuit, which includes one or more working gas conduits, which are formed in the platelet manifold proximate to the combustion chamber so that heat is transferred from the combustion chamber to the working gas flowing within the working gas conduits.
This platelet construction advantageously permits precision fabrication of the conduits and combustion chamber in the manifold. This results in more efficient heat transfer to the working gas as the overall size of each individual combustion chamber and each working gas circuit is substantially reduced in comparison with conventional manifolds due to the design of the present invention. More specifically, instead of having one large burner device with one combustion chamber and 36 or so working gas circuits (heater tubes) per piston cylinder, the manifold of the present invention has a substantially greater number of individual combustion chambers, e.g., over 100 and preferably over 200 per piston cylinder, as well as over 100 hundred working gas circuits. As a result, the dimensions of each combustion chamber and each working gas circuit are substantially reduced and may be precisely tailored using platelet technology. This results in a reduction of dead volume in each working gas circuit, improved heat transfer from the combustion chamber to the working gas, and improved efficiency of the combustion process performed in the combustion chamber.
In another aspect of the present invention, platelet technology is used to incorporate the internal region of the Stirling displacer cylinder head end into a platelet stack, which provides multiple heat exchangers. In a first aspect, the cylinder head end has working gas channels and ports formed therein to permit the working gas to flow to and from the cylinder head end region. By forming the working gas channels in the head end, an even more effective and efficient heat transfer surface area is provided and this results in a more compact and lighter weight Stirling engine. In another aspect, the present invention provides an integrated structure in which all of the major parts of the head end of the Stirling cycle engine are integrated into one cylindrical platelet device. The use of very small platelet coolant passageways makes possible small, yet highly efficient heat exchangers. In other words and according to one embodiment, channeled platelet members are annularly arranged to form a piston chamber and also provide all of the heat exchangers for the head end.
In yet another aspect of the present invention, a multi-stage combustor for use in the Stirling engine is provided and may or may not include inter-stage cooling. The combustors of the present invention are able to reduce the emission of NOx by having a first combustor which operates at fuel rich or stoiochiometric conditions (low NOx emission) and a second combustor which introduces secondary air to dilute the combustion gases and reduce the combustion temperature while maintaining the NOx emission at low levels. High system performance is still maintained.
In yet another embodiment, the head end of the Stirling engine includes a working gas heat exchanging plate which is bonded on top of a platelet manifold which is itself coupled to the head end of the piston cylinder. The platelet manifold includes a number of channels, which receive the working gas and serve to both distribute the working gas to the heat exchanging plate and also provide communication ports to the piston chamber, so that the working gas may flow into and out of the piston chamber. The heat exchanging plate has a number of heat transfer passageways to efficiently heat the working gas and to provide metal cooling capability. The heat exchanging plate is in fluid communication with the platelet manifold so that the heated working gas flows into and out of the channels of the manifold. The working gas is heated as it flows through the plate because one surface of the plate is in direct contact with the hot combustion gases formed during the combustion process and actually, the plate partially forms the combustion chamber.
A platelet air injector is provided and is a platelet manifold for unburned combustion air and acts to simultaneously cool the air manifold platelets and preheat the incoming combustion air. The platelet air injector has a number of swirler orifices formed therein for injecting air into the hot combustion gases as they flow from the combustion chamber. The air is aimed at an upper surface of the heat exchanging plate to enhance combustion mixing and aid in the heat transfer between the hot combustion gases and the plate. This embodiment utilizes multi-staged micro-combustion for burning the fuel rich gas to completion resulting in many advantages described hereinafter.
Other features and advantages of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.